The present invention relates to a method and apparatus for measuring the thickness or the refractive index of a transparent film on a substrate. The invention is particularly useful in measuring thickness variations of a photoresist film on a semiconductor substrate, and is therefore described below with respect to this application.
A typical semiconductor manufacturing process uses more than 17 photolithography steps. In each step, a photoresist (PR) material is deposited on the semiconductor (e.g., silicon wafer) surface, and an optical process is used to copy patterns onto the PR. The patterned PR is then used as a masking layer for subsequent process steps, such as etching, implanting, depositing, scribing, grinding, etc.
A photolithography process includes the following steps:
a) film, wherein the PR layer is coated uniformly over the wafer,
b) baking, wherein the PR is baked at a moderate temperature to dry the solvents therein;
c) exposing, wherein the wafer is exposed through a mask in which the required pattern appears as a non-transparent printing;
d) developing, wherein a chemical process is applied to remove the exposed PR from the wafer, and.
e) post exposure baking, wherein the wafers are baked after the exposure in order to harden the photoresist.
The photolithography process is one of the most challenging technologies used in the semiconductor industry. The patterns printed in the critical layers set the dimensional limitations for the entire technology. The minimum line width achieved today in production is 0.18 xcexcm (1/1,000 mm); the next generation of technologies is expected to require minimum line width of 0.1 xcexcm and below.
To achieve the minimum line width with high uniformity within each wafer and from wafer to wafer, it is most important to control each and every parameter in the photolithography process. Exposure energy, PR chemical composition, development time and baking temperature are only a few of the parameters that can affect the final critical dimension (CD) and its uniformity.
One of the most critical parameters to be controlled is the PR thickness. Because of the optical nature of photolithography, fluctuations of several nano-meters in the layer thickness can have a substantial effect on the final CD.
In the current processes, the PR film is applied on a high speed spinning chuck, called a xe2x80x9cspinnerxe2x80x9d. The wafer is placed on the spinning chuck and is rotated at low speed while the PR is dispensed at the center of the wafer. After dispensing, the chuck is rotated at a high speed (300-5000 RPM). The centrifugal forces acting on the PR cause the PR liquid to flow towards the edge of the wafer. Most of the PR (ca. 95%) is spilled off the wafer and is collected in a bowl to be drained later. The adhesion forces between the wafer surface and the PR hold a smaller amount of the PR on the wafer. The final thickness is a function of the centrifugal forces, the adhesion to the surface, and the shear forces caused by the viscosity of the liquid. The viscosity is increased during the spin due to the solvents evaporation therefore the solvents evaporation rate affects also the final thickness.
To control the final thickness one should control the rotational speed, the ramp-up speed (xe2x80x9caccelerationxe2x80x9d), the PR viscosity, and the environmental conditions within the bowl, among other parameters.
After spinning, the wafer is transferred to sit on a hot plate to perform the pre-exposure bake. Solvent evaporation during the bake further reduces the PR thickness. Nonunifornity of the hot plate temperature can cause nonuniformity of the final thickness.
Today, the process is set up and controlled by running a bare silicon test wafer and measuring the final thickness, and thickness uniformity, by a stand-alone layer-thickness measurement device. In order to ensure process stability, a periodic test is done on the test wafer. If the thickness drifts out of the control limits, the production is stopped and corrective actions are taken to bring the final thickness back to target.
The above-described thickness monitoring procedures are inefficient and wasteful. They can cause serious delays in reacting to a process out of control (OOC)-Test wafers and PR, as well as operational and engineering time, are spent in running the tests in a stand-alone system. Machine operational time is also wasted when the processing of production wafers is held up until positive test results are obtained. If the test results are negative, reworking may be required, or a complete batch may have to be scrapped.
Moreover, in contrast to the test wafers used for measurement purposes, the real production wafers have a much more complicated topography below the PR layer. As can be seen from a comparison of FIG. 1a to FIG. 1b, there may be a large difference in the thickness at different places (d1-d2), which differences are not reflected in a flat test wafer measurement.
In addition, since most of the PR ends up in the bowl, a considerable quantity of PR material is wasted. PR is one of the most expensive materials in the semiconductor process and will probably be even more expensive when deep-UV lithography is implemented in future production processes.
An object of the present invention is to provide an improved method and apparatus for measuring the lateral variation of the thickness of a transparent film on a production wafer. Another object is to provide a method and apparatus which are particularly useful in measuring lateral thickness variations in photoresist films on semiconductor substrates to avoid many of the above described drawbacks in the existing method and apparatus.
According to one broad aspect of the present invention, there is provided a method of measuring lateral variations of a property, selected from the group consisting of thickness and refractive index, of a transparent film on a substrate, including the steps of: (a) illuminating the film with a beam of light of multiple wavelengths; (b) detecting the intensity of the light reflected from the transparent film for each wavelength; (c) producing a signal defining the variation of the intensity of the detected light as a function of the wavelength of the detected light; (d) decomposing the signal into principal frequencies thereof; and (e) determining from the principal frequencies the lateral variations of the property of the transparent film.
According to another broad aspect of the present invention, there is provided an apparatus for measuring lateral variations of a property, selected from the group consisting of thickness and refractive index, of a transparent film on a substrate, including: (a) an illuminating device for illuminating the film with a beam of light of multiple wavelengths; (b) a detector for detecting the intensity of the light reflected from the transparent film for each wavelength; and (c) a processor for: (i) producing a signal defining the variation of the intensity of the detected light as a function of the wavelength of the detected light, (ii) decomposing the signal into principal frequencies thereof, and (iii) determining, from the principal frequencies, the lateral variations of the property of the transparent film.
According to another aspect of the present invention, there is provided, in a process for fabricating integrated circuits on a wafer, wherein trenches are formed on a surface of the wafer and the surface then is covered with an oxide layer that fills the trenches, a method of measuring depths of the trenches, including the steps of: (a) illuminating at least a portion of the oxide layer with a beam of light of multiple wavelengths; (b) detecting the intensity of the light reflected from the oxide layer for each wavelength; (c) producing a signal defining the variation of the intensity of the detected light as a function of the wavelength of the detected light; (d) decomposing the signal into principal frequencies thereof; and (e) determining the depths of the trenches from the principal frequencies.
According to further features in the preferred embodiment of the invention described below, the film is a transparent coating on a semiconductor substrate, preferably on a die among a plurality of dies carried on a semiconductor wafer. As will be described below, a number of advantages are obtained when the beam of light is large enough to cover a complete die.
According to another aspect of the present invention, there is provided apparatus for measuring the thickness of a transparent film on the substrate in accordance with the above method.
As will also be described below, the method and apparatus of the present invention are particularly useful for measuring thickness variations or refractive index variations of a film in situ, and in real time, e.g., to control the deposition or removal of a layer on a semiconductor substrate.
Further features and advantages of the invention will be apparent from the description below.